Alpha-substituted polymers such as polymerized methacrylates, methacrylonitriles, and alpha-methylstyrene are thermally unstable and are known to depolymerize to their corresponding monomers at temperatures greater than .about.180.degree. C., depending upon the molecular weight of the polymer. Above 300.degree. C., poly(methyl methacrylate)(PMMA) depolymerizes rapidly at high conversions (about 95% or more). Typical extruding and molding temperatures for such polymers are 200.degree.-290.degree. C. Significant depolymerization to the respective monomers would occur in this temperature range, affecting the safety of the operation as well as the properties of the resulting product. Polymethacrylates, and in particular PMMA, are the alpha-substituted polymers most widely used in commercial applications. In order to broaden the range of applications for these polymers it is critical to improve their thermal oxidative stability.
In the manufacture of grafted polymers comprising a backbone of a propylene polymer material, to which is grafted PMMA, small amounts of monomers such as methyl acrylate, butyl acrylate and styrene are typically copolymerized with the methyl methacrylate to improve thermal stability, since polymers of these monomers are much more stable to heat and undergo degradation at relatively higher temperatures. However, addition of these monomers affects the mechanical properties of the resulting grafted polymers as well as the molecular weight of the polymers and the efficiency of the graft polymerization reaction.
There is still a need for a method to increase the thermal oxidative stability of grafted polymers that include polymerized acrylic monomers, compared with that achievable with the comonomers currently used for this purpose.